This invention relates generally to earth drilling hydraulic shock absorbers and more particularly to an earth drilling hydraulic shock absorber having a sealed lubricated reciprocating anvil.
U.S. Pat. No. 4,067,405 and U.S. Pat. No. 4,171,025 disclose earth drilling hydraulic shock absorbers having a reciprocating anvil 26. The anvil 26 includes a spline connection as best shown in FIG. 6 which reciprocally mounts the anvil with a casing while preventing rotation of the anvil relative to the casing. Drilling fluid flowing through the shock absorber is allowed to flow through the spline connection 28 and out the sloping crossbores 44 and through the drill bit. It has been found in practice that the abrasive materials which collect in the drilling mud tend to wear out the spline connection 28 as the tool continually reciprocates in operation. In certain formations, depending on the abrasive materials, the operating life of the shock absorber may be greatly decreased due to excessive wear on the spline connection 28.
It is thus desirable to seal the spline connection of the anvil from the drilling fluid which contains the abrasive elements to prevent the excess wear.
It would also be desirable to provide a lubricating fluid around the spline connection to further reduce wear on the splined portion. However, the earth drilling hydraulic shock absorber of the invention is typically used many thousands of feet below the surface where it encounters significant hydrostatic pressure due to the drilling fluid which is present in the borehole and within the drill string and hydraulic shock absorber. When a sealing means is provided which seals the spline connection of the anvil, pressure differentials across the seal means may quickly cause failure of the seal means as the pressure across the seal means attempts to equalize with the hydrostatic pressure in the borehole.
One means of equalizing pressure across the seal means is through the use of a moving compensator or piston which substantially mantains a lubricating fluid within the tool at the same pressure on one side of the seal means as the hydrostatic pressure of the drilling fluid in the shock absorber and borehole on the other side of the seal. While such a seal means will provide the pressure equalization, it also requires an additional moving part and may result in an increased manufacturing cost and operating cost.
An object of the invention is to provide a sealed lubricated unit which will retain the lubricating fluid within the tool and prevent exposure of the spline connection of the tool with the abrasive drilling fluid. Another subject of the invention is to equalize pressure across the sealing means as the hydrostatic pressure changes to protect the seal means from damaging differential pressures other objects of the invention will be apparent from the following specification. As far as known, heretofore no one else has achieved these results.
In certain situations, a shock absorber which utilizes the shock-absorbing effect of the plurality of pistons at lower downweights of the drill string is preferable. This allows the anvil to initially engage the first piston at lower downweights exerted on the drill bit particularly when very high pressure drilling fluid is utilized. In the example given in U.S. Pat. No. 4,067,405, approximately 18,360 lbs. of down force is required to engage the anvil with the first piston. Reducing the effective piston area of the anvil will reduce that force so that the anvil will support less weight until it engages the first piston.
An object of the invention is to accomplish this result in an economical manner, by making a shock absorber readily convertible from a high pressure tool (i.e. 18,360 lbs.) to a low pressure tool (some lesser force). This can be achieved by the invention which provides a unique contribution which can reduce inventory and manufacturing costs.